In direct injection spark ignition engines, there are two modes of operation that are typically used. The first mode is termed stratified mode where fuel is injected during the compression stroke of the engine. In the stratified mode of operation, the air/fuel ratio is operated lean of stoichiometry. In the second mode of operation, termed homogeneous operation, fuel is injected during the intake stroke of the engine.
During homogeneous operation, the air/fuel can operate either lean or rich of stoichiometry. However, in some circumstances, the operable stratified operation range of lean air/fuel ratios does not coincide with any operable homogeneous, lean air/fuel ratio. Therefore, when switching between these two modes of operation, air/fuel ratio from one cylinder event to the next cylinder event changes in a discontinuous way. Because of this discontinuous change in air/fuel ratio, engine torque is uncompensated, and has an abrupt change.
One method for eliminating abrupt changes in engine cylinder air/fuel ratio is to adjust ignition timing so that abrupt changes in engine torque will be avoided. Another solution is to adjust throttle position to reduce or increase fresh charge flow entering the intake manifold and therefore compensate for changes in engine torque during discontinuous cylinder air/fuel ratio changes.
The inventors herein have recognized disadvantages with the above approaches. Regarding ignition timing adjustments to avoid abrupt changes in engine torque, this method is only applicable when the magnitude of the torque change is small. In other words, the range of authority of ignition timing is limited by engine misfire and emission constraints. Therefore, the approach is not generally applicable.
Regarding throttle position adjustments to prevent abrupt changes in engine torque, controlling flow entering the manifold cannot rapidly control cylinder charge due to manifold volume. In other words, air entering the cylinder is governed by manifold dynamics and therefore there is a torque disturbance when using the throttle to compensate for discontinuous cylinder air/fuel ratio changes. For example, if the throttle is instantly closed and no air enters the manifold through the throttle, cylinder air charge, does not instantly decrease to zero. The engine must pump down the air stored in the manifold, which takes a certain number of revolutions. Therefore, the cylinder air charge gradually decreases toward zero. Such a situation is always present when trying to change cylinder charge using a control device such as a throttle.